The invention relates to a device for batch feeding a fluidizable particulate material from a silo-having at the bottom thereof a closable, conical outlet into a reaction vessel and in particular, alumina from a day's storage silo to a break in the crust on an electrolyte cell used in manufacturing aluminum by the fused salt electrolytic process.
The production of aluminum by the fused salt electrolytic reduction of aluminum oxide involves the latter being dissolved in a fluoride melt comprised for the greater part, of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum itself forming the actual cathode. Dipping into the melt from above are anodes which, in conventional processes, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen forms at these carbon anodes and combines with the carbon to form CO.sub.2 and CO. The electrolytic process takes place at a temperature in the range of approx. 940.degree.-970.degree. C.
During the course of the electrolytic process the electrolyte becomes depleted in aluminum oxide. At a lower concentration of 1-2 wt% aluminum oxide in the electrolyte an anode effect occurs whereby the voltage increases for example from about 4-5 V to 30 V or more. At this time the crust of solid electrolyte material should be broken open and the concentration of aluminum oxide increased by adding alumina.
Under normal production conditions the cell is normally fed alumina at regular intervals even when no anode effect occurs. In addition, each time the anode effect occurs, the crust is broken open and the alumina concentration increased by addition of aluminum oxide. The foregoing constitutes servicing of the cell.
For many years now servicing the cell has included breaking open the crust between the anodes and the sidewall of the cell and adding alumina there. This practice has met with increasing criticism because of pollution of the air which occurs in the pot room and the surrounding area. In the case of hooded pots, maximum capture of the pot fumes can be achieved only if the servicing of the cell is automated. After breaking open the crust, the alumina is added either locally and continuously according to the point feeder principle or discontinuously over the whole of the longitudinal or transverse axis of the cell.
The known alumina storage bunkers or silos mounted on the electrolyte cells are in the form of funnels or containers with a funnel-shaped or conical outlet in the lower part. The storage content of the silo on the cell meets the cell requirements for one to two days, and can therefore be called day's storage silo.
The feeding of alumina from the silo to a break in the crust on the molten electrolyte takes place in known devices by opening a flap which is lowered for cell charging purposes, or by means of other systems employing feeding screws, pistons or the like.
These feeding devices have the disadvantage that mechanically moveable parts must be built into the reduction cell. Consequently they are exposed to the heat and dust of the cell atmosphere which makes maintenance necessary to a greater or lesser extent. In many versions there is the danger of mechanical damage, in particular during the changing of the anodes.
It is therefore the object of the inventors to develop a device for continuous controlled feeding of a fluidizable particulate material, with no mechanically moveable elements and in the form of a compact, robust unit which can be built into a silo, and such that due to its simple construction the said device is economic to manufacture and, to a large extent, maintenance free.